Dance flies attract males with their hairy legs and inflatable sacs

Female dance flies with large inflatable sacs and hairy legs are more attractive to males.

Dance flies get their name because they form aerial mating swarms and seem to dance together in the air. It looks simple and beautiful, but it turns out to be as complex as any ceilidh.

Rosalind Murray and colleagues at the University of Toronto Mississauga and the University of Stirling have now revealed the lengths to which females of the species Rhamphomyialongicauda go to attract a mate at these dances.

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Before they enter a swarm, females swallow air to inflate sacs along their abdomen to make their bodies look bigger. They also keep their legs parallel to their abdomens. Because of the hair-like pinnate scales along the legs, this too makes the females look larger to males entering the swarm from below.

“Females look like little helicopters, flocking together within the swarm,” says Murray. “The males assess the females, then they pair off, and basically fall out of the sky.”

The hairier the better

The team has found that males find females with big sacs three times as attractive as those with small sacs. But if the sacs are small, the hairier the female’s legs are, the better.

Positioning is important too. The prime spot seems to be in the centre of the swarm, perhaps partly because spiders lurk around the periphery.

The team suggests that the sacs are a newer evolutionary sexual weapon than the hairs, because fewer species of dance flies have them and their effect on males is so strong.

“You typically see males, like peacocks, with ornaments,” says Murray, but very rarely in nature do females rely on ornaments to attract a partner.

So why are they doing this? By making themselves look bigger, the team suggests, the females appear full of eggs and a good mate choice.

This could all just be for the hook-up. But females often mate more than once. Perhaps they strive to attract males as a way to get additional food to allow them to produce more eggs, says Murray. Each male comes to the dance bearing a nuptial gift of a meal – usually other insects or even other males of the same species.

“The suggestion that females are using dishonest signals to attract males turns many mate-choice models on their head,” says Devin Arbuthnott at the University of British Columbia – Vancouver.